1. Field of the Invention
The present invention relates to a switchable magnetic gradient coil system that is formed on the basis of saddle coils that are operated as primary coils or auxiliary coils for different gradient axes.
2. Description of the Prior Art
The required efficiency of a magnetic gradient coil essentially depends on the type of MR-imaging being conducted. Conventional MR-imaging normally requires a good linearity volume (xcx9c5% in the linearity volume of 40-50 cm) given moderate gradient intensities (10-20 mT/m) and switching times (xcx9c1 ms). High gradients (20-40 mT/m) are extremely rapidly switched (100-500 xcexcm) the fast MR-imaging. Side effects in the form of peripheral muscle stimulations can occur as described, for example, by Budinger et al., xe2x80x9cPhysiological Effects of Fast Oscillating Magnetic Field Gradients,xe2x80x9d J. Comp. Assisted Tomog., Vol. 15, 1991, pp. 909-915. The linearity volume of the gradient coils is generally reduced in order to avoid these effects, since this produces a reduction of the maximum field boost and therefore also a reduction of the stimulation risk (apart from other aspects, the maximum field boost determines the stimulation risk). Therefore, the linearity volume can be reduced from typically 40-50 cm to 20 cm DSV given fast gradient coils. Normally, a coil with such properties is not appropriate for conventional whole body applications but is appropriate for fast MR-imaging techniques, such as EPI, RARE, HASTE, GRASE etc. Speed is the significant advantage here.
Another reason for different field qualities is that the linearity volume normally becomes reduced with increased distance from the center when a gradient coil is designed for a specific volume. The human body, however, does not necessarily conform to this shape. For example, the shoulders are situated in this area. Given pickups of the spine, it is often, expedient to image the entire spine without rearrangement. Depending on the center positioning, the cervical and/or lumbar vertebras may also lie in the area of the higher non-linearities. Image distortions cannot be avoided as a result. In head gradient coils, the homogeneity volume is smaller due to the smaller diameter of the coil. This allows parts of the brain to be imaged but does not allow imaging of the cervical spine. Therefore, it can be desirable for the radiologist to change from a central FOV to a displaced or shifted FOV. Heretofore, this has not been possible. There are only embodiments of one type or the other type.
Due to the above reasons, the customer needs to decide whether a field quality A, B or C is needed. It would be desirable, however, to have a number of coil properties (field qualities) combined in one coil and to activate these depending on the application. A basic problem in achieving this is the accommodation of the multiple coils in the coil body without significantly increasing the volume (thereby causing the coils to be more expensive) and without comprising the sub-coil properties that are partially competing.
German OS 195 40 746, describes a modular gradient system in which a conventional gradient coil system and a rapid gradient coil system are combined in a coil body. In this known system the conventional gradient system exhibits a large linearity volume but can be switched only slowly and also allows only average gradient amplitudes. By contrast, the rapid gradient system shows a smaller linearity volume but allows extremely high gradient amplitudes to be switched more rapidly.
For appropriate background, the general state of the art of gradient coil technology will be summarized.
Shielded and unshielded gradient coils are utilized in the MR-imaging. Generally, these gradient coils are sealed or cast in the vacuum, so that a higher voltage load is achieved, Therefore, the gradient coil is formed as a compact tube with typically three electrical terminal pairs, given cylindrical MR systems, or is formed as a plate pair given C-shaped MR systems, (such as the Siemens MAGNETOM OPEN) with typically six electrical terminal pairs. The subject matter disclosed herein is only concerned with cylindrical systems, however, the discussion is generally valid for non-cylindrical systems as well.
Generally, non-shielded gradient coils are composed of a number of sub-coils that are connected in the coil body. For example, an unshielded transverse coil (X, Y) is composed of four coils referred to as saddle coils (FIG. 1), which are connected to a gradient axis in the coil body.
In principle, actively shielded gradient coils are composed of primary layers and secondary layers. Primary coils and secondary coils are matched to one another such that the magnetic flow is minimized on a selected cylinder surface (normally the position of the cryostat tube of the magnet or other conductive structures) outside of the coil tube in order to avoid eddy currents. The sub-coils also are connected within the coil body, so that connection to the exterior world is normally only via the six terminal contacts described above.
Coils referred to as split gradient coils represent an exception; symmetrical sub-coils are separately connected to the amplifier. Such a coil is normally comprised of two symmetrical parts. The terminals are doubled as a result. The advantage of such coils is the faster switching times (due the halved inductivity), however, twice as many gradient amplifiers are required making the system more expensive.
Segment coils are known and represent a further realization of gradient coil designs. Examples of such segment coils are described in Genan OS 40 29 477 (corresponding to U.S. Pat. No. 5,198,769) and German PS 195 27 020 (corresponding to U.S. Pat. No. 5,675,255). No return conductors are disposed in the same cylinder surface as the active conductors in this type of coil design. The return conductors are situated at a different radius. Ideally, the segments exhibit an aperture or coverage angle of 120xc2x0 in order to achieve the best field quality. At least four segments are also connected to one unit within the coil body in order to generate a satisfactory gradient field. For adapting the linearity volume to the clinical requirements, a number of these segments are distributed along the Z-axis and are connected to one another, so that the coil body as such only has the usual three terminal pairs.
Coil designs referred to as 3D gradient coil designs are a mixture of the conventionally shielded coils and the segment coil. Given this design, some or all return conductors of the transverse coils are foregone, which would be situated at the ends of the gradient coil body. Instead of the return conductors, direct connections between the primary layer and the secondary layer are present.
All these known switchable gradient coils have the disadvantage of either requiring an extremely complicated drive for switching the different sub-windings or, due to the possibility of being able to only accommodate the hitherto utilized saddle coils as pairs in a radial layer, a number of radial layers is required for purposes of accommodating the sub-coils, resulting in the volume of the gradient coil being significantly increased and the field properties being impaired in the imaging region.
An object of the present invention is to provide a switchable gradient coil system formed by saddle coils such that a gradient coil system having a small volume can be realized with a construction that is simple in terms of switching, and which can be optionally designed for different performance features.
The object is inventively achieved in a gradient coil system composed of saddle coils that are operated as primary coils or auxiliary coils respectively for different gradient axes, and wherein the auxiliary coils, which exhibit a correspondingly reduced subtended angle compared to the coils operated as primary coils, for one gradient axis are arranged in the gaps between coils that are diametrically offset in a radial plane for the other gradient axis, and wherein the primary coils and/or auxiliary coils of each layer can be divided into a number of sub-coils that can be connected differently with one another.
In a first embodiment of the invention the auxiliary coils for both gradient axes are arranged as pairs that are offset to one another by 90xc2x0 in a separate radial planes, with each auxiliary coil subtending having an angle xcex1xe2x89xa690xc2x0. Therefore, two radial layers are no longer necessary for the auxiliary coils, since they are accommodated together in one radial layer.
In a second embodiment of the invention the auxiliary coils for each gradient axis are arranged in the same radial layer between the primary coils for the other gradient axis, these primary coils each exhibiting a subtended angle xcex1xe2x89xa790xc2x0, preferably 120xc2x0.
As a result, additional radial layers, which only increase the spatial construction of the gradient coil system, are not required at all in this embodiment for accommodating the auxiliary coils. The space present between the primary coils, which has hitherto not been used, is utilized in the inventive system to accommodate auxiliary coils for the other gradient axis. The fact that the additional coilsxe2x80x94due to the preferred fashioning of the primary coils with a subtended angle of 120xc2x0xe2x80x94can then only exhibit overlap angles of maximal 60xc2x0, does not constitute a limiting factor, since they need not be solely responsible for producing the desired gradient coil for qualities.
This inventive embodiment results in a total structure which occupies a small space and also enables an extremely simple connecting of the different primary coils and auxiliary coils among each other, even when the inventive switchable gradient coil is designed in a shielded fashion and coil pairs for the same gradient axis being in different radial layers. Given this shielded embodiment, auxiliary coils are arranged in at least one of the layers for another gradient coil axis.
The coil pairs of each layer, which are respectively operated as primary coils for one gradient axis and as primary coils or auxiliary coils for the other gradient axis, are connectable to the auxiliary coils or primary coils arranged in other layers in different ways depending on the desired performance features. The primary coils and auxiliary coils can be statically connected with one another prior to executing the imaging sequence and can be dynamically connected during the imaging sequence, thereby allowing the field properties and performance features of the switchable gradient coil to be changed during a sequence. Suitable connection arrangements for this purpose are described in U.S. Pat. Nos. 6,205,140 and 6,297,635. The teachings of each of these patents are incorporated herein by reference.